1. Field of the Invention
The present invention relates to a semiconductor laser element and a method for adjusting self-induced oscillation intensity of the same. In particular, the present invention relates to an improvement of the electrical and optical characteristics of a high-power and a low-power semiconductor laser elements having a low noise characteristic used for optical disks and the like.
2. Description of the Related Art
Semiconductor lasers are widely used as light sources of optical disk apparatuses. In the case where a signal is recorded onto an optical disk, a high-power semiconductor laser having a light output of 30 mW or more is used. The high-power semiconductor laser is also used for reproducing the signals.
The high-power laser for recording and reproducing signals is required to have the following performance.
(1) First, noise induced by a return light from the optical disk or any other optical parts while the signals are reproduced is to be low.
(2) Second, ellipticity of light emitted from the laser element (emitted laser light) is to be small so as to enhance optical coupling efficiency with a collimator lens. The ellipticity of emitted laser light herein represents a ratio (.theta..perp./.theta.&lt;) of a half width .theta..perp. of a divergent angle (radiation angle) of the emitted laser light in a direction perpendicular to an active layer in a face perpendicular to a direction of length of resonator of the laser element to a half width .theta.&lt; of a divergent angle (radiation angle) of the emitted laser light in a parallel direction to the active layer of the laser element. More specifically, the ellipticity (.theta..perp./.theta.&lt;) is 2.5 or less. The above half width .theta..perp. depends on the thickness of the active layer of the laser element and a composition ratio of a cladding layer. Since the value of the above .theta..perp. is within the range of 20.degree. to 28.degree., .theta.&lt; is required to be 11.degree. or more.
(3) Third, a driving current and a driving voltage should be as low as possible so as to lower electric power consumed by the semiconductor laser element.
As for the noise of the laser element described in (1), since a conventional high-power laser has an oscillation longitudinal mode which is a single mode and has high coherence in terms of time, noise is likely to be induced by the return light. In order to avoid this problem, the following high-frequency superimposing method is adopted. The driving current is superimposed by a high-frequency signal during reproduction of the signal so as to alter the oscillation mode of the laser into a longitudinal multimode, thereby lowering the coherence of time. As a result, the effect of the return light is reduced.
However, the high-frequency superimposing method requires a high-frequency circuit. Therefore, there is a problem in miniaturizing a driving circuit and lowering the consumption power of the apparatus. Studies utilizing a self-induced oscillation phenomenon have been extensively pursued so that the noise can be reduced without using a high-frequency superimposing circuit (Reference: Singaku Giho OQE (Optical and Quantum Electronics) 88-5, Tanaka et al).
In the state of self-induced oscillation, although the laser is driven by a constant current, the intensity of the laser light is not constant but varied along with time in high-frequencies of 1 GHz or more. As described above, when the intensity of the emitted laser light is varied along with time, the width of the oscillation wavelength is increased and the oscillation spectrum is made to have a multimode, thereby lowering the coherence of the emitted laser light. As a result, the interference between the emitted laser light and its return light is restrained in order to reduce the noise.
Hereinafter, a structure of a conventional high-power semiconductor laser element which is designed so as to assure self-induced oscillation will be described.